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Abstract

Cave deposits containing sediments of eroded soils and relict soils preserved on
modern uplands provide some of the few insights into understanding the Quaternary
evolution of central Texas landscapes and environments. This study investigates an
ancient soil-erosion event on the Edwards Plateau of central Texas by using a new
approach that relies on the strontium isotope composition of the ancient soil as an
indicator of ancient soil thickness. I found the strontium isotope composition of soil on
the Edwards Plateau today varies with soil thickness. I also found that variations in
strontium isotope compositions of fossil plants and animals provide a record of temporal
changes in soil thickness. Furthermore, I present stable carbon, oxygen, and nitrogen
isotope data measured on sediments and fossils from a central Texas cave-fill deposit that
support the hypothesis that increases in aridity and increases in precipitation intensity
may have facilitated massive, late Quaternary soil erosion on the Edwards Plateau.
The origin of the former thick soil cover that was removed by the erosion is
poorly understood. This study uses the geographic distribution and the textural and
mineralogical properties of relict thick soils along with their neodymium isotope
compositions, and rare earth element concentrations to determine the silicate source of
the former thick soils. I propose that the Del Rio Clay, a locally-eroded,
stratigraphically-higher, clay-rich strata provided the silicates to form thick soils
overlying the silicate-pure Edwards Limestone.
Four important conclusions of this study are that: 1) in areas where the soil and
bedrock are distinct in their strontium isotope composition, the strontium isotope
composition of the soil varies with soil thickness, 2) the strontium isotope composition of
terrestrial fossils can be a useful tool to reconstruct temporal changes in soil thickness, 3)
climate variability can facilitate massive soil erosion with regional environmental
consequences, and 4) soil formation from now-eroded bedrock of a higher stratigraphic
unit may be more common than previously recognized. Thus, in light of future changes
in climate and land use, soil conservation strategies should be considered in regions
where soils rest on relatively pure limestones, because they are likely a “non-renewable”
resource.